Compact toothed wheel gearing with large gear ratio

ABSTRACT

A compact gear system that includes an input shaft (28), an output shaft (1) and gear wheels which drivingly connect the same, wherein the output shaft (1) is journalled in a gear housing (2). The output shaft (1) is hollow and has pins (5a) which project radially outwards in a normal plane and each of which carries a gear wheel (6). Provided on each side of the pins is a respective crown wheel (11, 12) journalled on the output shaft (1) and having a toothed ring (17, 18) in engagement with the gear wheel (6). Each crown wheel also has a cylindrical toothed ring (19, 20) which coacts at different transmission ratios with a respective toothed ring (25, 26) on an eccentric shaft (21) which extends parallel with the output shaft (1) and which is rotatably journalled in the housing (2), wherein the eccentric shaft (21) is driven by the input shaft (28). As the input shaft (28) rotates, the pins are forced to rotate about the center axis of the output shaft (1) and therewith entrain the output shaft with high reduction. The compact gear system is particularly intended for rotating the arm of an industrial robot in relation to the robot frame.

This application is a continuation of U.S. application Ser. No.08/983,014; filed Oct. 5, 1998 now U.S. Pat. No. 5,993,348.

The present invention relates to a compact gear system. A compact gearsystem is constructed to provide a high transmission ratio between aninput drive shaft and an output shaft within a small space. Suchapplication is required in the case of industrial robots with which arobot arm is pivotal in relation to a frame and the pivotal movement isachieved with the aid of a drive motor whose rotational speed must begeared down considerably without the assistance of over-bulkytransmission devices.

U.S. Pat. No. 1,802,112 teaches a differential type of reduction gearwith which very high transmission ratios can be achieved. This reductiongear includes a relatively large number of gear wheels, all of which areconical and require input and output shafts to be mutuallyperpendicular. Such reduction gears incur high manufacturing costs, takeup a relatively large amount of space, and are difficult to adapt tosuit different applications.

SE 500 626 teaches a similar gear in which some of these drawbacks arealleviated. This gear, however, also has certain drawbacks. Although theoutput shaft is hollow, a transverse shaft included in the gearpartially blocks the shaft bore and therewith makes it difficult toutilize the space in the hollow shaft. Because it may be advantageous,particularly in the case of industrial robot applications, to use thehollow shaft as ducting for electric cables, different media such asair, water and paint, and other drive shafts, the obstructive transverseshaft constitutes a disadvantage. Furthermore, it is necessary for theinput shaft to define a given small angle with the normal plane of theoutput shaft, which increases the precision required and makes assemblymore complicated.

The object of the present invention is to provide a compact gear systemof the aforesaid kind in which those drawbacks associated with the knowngear systems are avoided.

This object is achieved in accordance with the invention with a compactgear system of the kind defined in the preamble of claim 1 and havingthe characteristic features set forth in the characterizing clause ofsaid claim.

Because input and output shafts are parallel and connected drive-wise inthe given manner, the gear system is simple to assemble, its housingrequires working from only one side, and the system is reliable inoperation and affords a very compact design. The transmission ratio canbe changed with the aid of relatively simple modifications. Furthermore,the gear system takes up none of the space in the hollow shaft, which isextremely beneficial as before mentioned.

It is advantageous to permit the drive gear wheel on the input shaft todrive directly one crown wheel, and to drive the other crown wheelthrough the medium of an eccentric shaft, therewith enhancing thepossibility of obtaining a compact and readily assembled design.

In another preferred embodiment, the gear housing is connected to arobot arm and the output shaft is connected to a frame structure, orvice versa, resulting in an application of the invention where itsadvantages are of great significance.

These and other advantageous embodiments of the invention will beapparent from the dependent claims.

The invention will now be described in more detail with reference to apreferred embodiment thereof and also with reference to the accompanyingdrawings, in which

FIG. 1 is a sectional view of an inventive compact gear system;

FIG. 2 is a schematic side view of an industrial robot fitted with aninventive gear system; and

FIG. 3 is a schematic end view of the industrial robot shown in FIG. 2.

FIG. 1 shows the output shaft 1 rotatably journalled in a gear housing 2by means of bearings 3. The shaft is hollow and has athrough-penetrating cylindrical bore 4 that can be used for the passageof electric cables, media, or other drive shafts. The output shaft 1includes a plurality of pins 5a, 5b, in the illustrated case two pins,which extend in a normal plane radially outwards and which are formedintegrally with the shaft 1. A conical gear wheel 6 is journalled inbearing 7 for rotation about each pin 5a, 5b, wherein the bearing issupported by a bearing seat 8 mounted on the pin 5a and held firm bymeans of a plate or washer 9. The plate 9 is secured to the pin by aradial screw 10. All of the conical gear wheels 6 journalled on the pins5a, 5b have mutually the same diameter and the same number of teeth.Although only one pin 5a and associated gear wheel 6 will suffice, twopins will provide greater reliability in operation when large moments offorce are anticipated, and in some applications three or four pins andassociated gear wheels may be necessary. A crown wheel 11, 12 isjournalled in a respective bearing 13 and 14 for rotation about theoutput shaft 1 on each side of the normal plane in which the pins 5a, 5bare disposed, said bearings being firmly held axially by a respectivelocking ring 15 and 16. Each of the two crown wheels 11, 12 has aconical toothed ring 17 and 18 respectively, and a respectivecylindrical toothed ring 19 and 20. The crown wheels 11, 12 are mutuallyidentical and their respective conical toothed rings 17 and 18 are inengagement with the two conical gear wheels 6.

Extending parallel with the output shaft 1 is a hollow eccentric shaft21 which is rotatably journalled in bearings 22, 23 on a shaft 24fixedly mounted to the gear housing 2. Mounted on each end of theeccentric shaft 21 is a respective cylindrical toothed ring 25 and 26which have mutually different tooth numbers, wherein in the case of theillustrated embodiment the toothed ring 25 located to the left of theFigure has more teeth than the other toothed ring 26. The toothed ring25 on the left of the Figure engages with the cylindrical toothed ring19 of the left-hand crown wheel 11, whereas the right-hand toothed ring26 on the eccentric shaft engages with a pinion wheel 27 on the inputshaft 28 connected to a motor 30. The pinion wheel 27 also engages withthe cylindrical toothed ring 20 of the right-hand crown wheel 12.

The illustrated gear system functions in the following manner. Thepinion wheel drives the right-hand crown wheel 12 directly through itsengagement with the cylindrical toothed ring 20 of the crown wheel, 12and drives the left-hand crown wheel 11 via the two toothed rings 26 and25 on the eccentric shaft 221. Because the left-hand toothed ring 25 onthe eccentric shaft 21 has more teeth than the right-hand toothed ring26, the left-hand crown wheel 11 will rotate at a slightly higher speedthan the right-hand crown wheel 12. When z₁ =the number of teeth on thepinion wheel 27, z₂ =the number of teeth on the right-hand toothed ring26 on the eccentric shaft, z₃ =the number of teeth on the left-handtoothed ring 25 on the eccentric shaft, and z₄ =the number of teeth onthe cylindrical toothed ring 19 and 20 of respective crown wheels 11,12, the following relationships will be obtained when n₁ denotes therotational speed of the input shaft 28.

The right-hand crown wheel 12 will rotate at the speed: ##EQU1##

The eccentric shaft will rotate at the speed: ##EQU2##

The left-hand crown wheel will rotate at the speed: ##EQU3##

The speed differential between the two crown wheels 11, 12 will then be##EQU4## two wheels 11, 12 rotate in mutually different directions.

The conical toothed rings 17, 18 of respective crown wheels 11, 12 arein engagement with the conical gear wheel 6 and as a result of saidspeed difference, the pin 5a on which the conical gear wheel 6 isjournalled will be forced to rotate about the centre axis of the outputshaft 1. Since the pin 5a (and the pin 5b) is fixedly connected to theoutput shaft 1, the output shaft will accompany rotation of the pin. Therotational speed will be half the speed difference between the speeds ofrespective crown wheels 11, 12, i.e. the output shaft 1 will rotate atthe speed: ##EQU5##

The transmission ratio will thus be: ##EQU6##

Transmission ratios in the order of 100:1 to 400:1 can be readilyachieved with a suitable combination of toothed ring tooth numbers. Forinstance, when z₁ =20, z₂ =33, z₃ =36 and z₄ is 150, the transmissionratio will be: ##EQU7##

FIGS. 2 and 3 illustrate how the inventive compact gear system can beused with an industrial robot, where in the illustrated case aninventive gear system is provided at a number of different places. Afirst compact gear system is arranged with its output shaft 1a connectedto the foot 34 of the robot, and with its housing 2a connected to theframe structure 32 for achieving pivotal movement of the frame structure32 about the foot 34, and is driven by the motor 30a. The lower arm 31of the robot is similarly pivotal in relation to the frame structure 32through the medium of the compact gear system driven by motor 30b.

The upper arm 33 of the robot is pivoted or rotated by a third compactgear system driven by motor 30c. This output shaft 1c is connected tothe upper arm 33 and its housing 2c by a lever 35 pivotally mounted onthe upper end of the lower arm 31. The lever 35 is swung about its pivotaxle in the lower arm 31 by a parallel strut 36 which is manoeuvered bya fourth compact gear system driven by motor 30d.

It will be understood that the invention is not restricted to theaforedescribed embodiment of a compact gear system. For instance thegear wheel 6 need not be conical, but may be cylindrical instead, e.g.of the kind marketed under the trademark Cylkro. Neither are the toothedrings 17 and 18 restricted herewith to a conical configuration.

What is claimed is:
 1. A compact gear system having a high transmissionration and including a gear housing (2), an input shaft (28), an outputshaft (1) journalled into the gear housing (2), and gear wheel meanswhich connect drive-wise the input (28) and output (1) shafts,characterized in thatsaid output shaft (1) is parallel with the inputshaft (28); said output shaft (1) has fixedly mounted thereon in anormal plane at least one radially outwards projecting pin (5a), whereina gear wheel (6) is rotatably journalled on each said pin (5a); first(11) and second (12) crown wheels are rotatably journalled on saidoutput shaft (1) on each respective side of each said pin (5a), whereineach crown wheel includes a first toothed ring (17, 18) and a secondcylindrical toothed ring (19, 20), and wherein the first toothed ring(17, 18) engages with each gear wheel (6); and an offset shaft (21),which extends parallel with an output shaft (1) and which is driven by apinion wheel (27) of said input shaft (28), includes first and secondtoothed rings (25, 26), wherein the first toothed ring (25) coacts withthe cylindrical toothed ring (19) of the first crown wheel (11) at afirst transmission ratio, and wherein the pinion wheel (27) of saidinput shaft (28) is in engagement with said second toothed ring (26) andwith the cylindrical toothed ring (20), thereby defining a secondtransmission ratio which is the ratio of the number of teeth of saidpinion wheel (27) and the number of teeth of said second toothed ring(26) and further defining a third transmission ratio which is the ratioof the number of teeth of said pinion wheel (27) and the number of teethof the cylindrical toothed ring (20), and wherein a ratio of said thirdtransmission ratio to said second transmission ratio differs from thefirst transmission ratio.
 2. A compact gear system according to claim 1in which the output shaft (1) is hollow and includes a continuous bore(4) through which electric cables, media-carrying conduits and otherdrive shafts can be passed.
 3. A compact gear system according to claim2 in which the output shaft (1) includes two radially outwardsprojecting pins (5a, 5b).
 4. A compact gear system according to claim 2in which the input shaft (28) is drivingly connected to the offset shaft(21) by virtue of the engagement of a pinion wheel (27) fixedlyconnected to the input shaft (28) with said second toothed ring (26),wherein said pinion wheel (27) is also in engagement with thecylindrical toothed ring (20) of the second crown wheel (12).
 5. Acompact gear system according to claim 2 in which the first (25) and thesecond (26) toothed rings have mutually different tooth numbers, whereinthe cylindrical toothed rings (19, 20) of respective first (11) andsecond (12) crown wheels have mutually the same tooth number, andwherein the first toothed rings (17, 18) of respective first (11) andsecond (12) crown wheels have mutually the same tooth number.
 6. Acompact gear system according to claim 2, wherein the offset shaft (21)is hollow and is rotatably journalled on a carrier shaft (24) byinternal bearings (22, 23), and wherein said carrier shaft (24) isfixedly connected to the gear housing (2).
 7. A compact gear systemaccording to claim 2, wherein either the output shaft (1b) or the gearhousing (2b) is connected to a robot frame (32) and the other to a robotarm (31).
 8. A compact gear system according to claim 2, wherein eitherthe output shaft (1c) or the gear housing (2c) is connected to a firstrobot arm (33) and the other to a lever (35) which is rotatably mountedon a second robot arm (31) and provided with a maneuvering strut (36).9. A compact gear system according to claim 1 in which the output shaft(1) includes two radially outwards projecting pins (5a, 5b).
 10. Acompact gear system according to claim 9 in which the first (25) and thesecond (26) toothed rings have mutually different tooth numbers, whereinthe cylindrical toothed rings (19, 20) of respective first (11) andsecond (12) crown wheels have mutually the same tooth number.
 11. Acompact gear system according to claim 9, wherein the offset shaft (21)is hollow and is rotatably journalled on a carrier shaft (24) byinternal bearings (22, 23), and wherein said carrier shaft (24) isfixedly connected to the gear housing (2).
 12. A compact gear systemaccording to claim 9, wherein either the output shaft (1b) or the gearhousing (2b) is connected to a robot frame (32) and the other to a robotarm (31).
 13. A compact gear system according to claim 9, wherein eitherthe output shaft (1c) or the gear housing (2c) is connected to a firstrobot arm (33) and the other to a lever (35) which is rotatably mountedon a second robot arm (31) and provided with a maneuvering strut (36).14. A compact gear system according to claim 1 in which the input shaft(28) is drivingly connected to the eccentric shaft (21) by virtue of theengagement of a gear wheel (27) fixedly connected to the input shaft(28) with said second gear wheel (26), wherein said gear wheel (27) isalso in engagement with the cylindrical toothed ring (20) of the secondcrown wheel (12).
 15. A compact gear system according to claim 14 inwhich the first (25) and the second (26) toothed rings have mutuallydifferent tooth numbers, wherein the cylindrical toothed rings (19, 20)of respective first (11) and second (12) crown wheels have mutually thesame tooth number, and wherein the first toothed rings (17, 18) ofrespective first (11) and second (12) crown wheels have mutually thesame tooth number.
 16. A compact gear system according to claim 14,wherein the offset shaft (21) is hollow and is rotatably journalled on acarrier shaft (24) by internal bearings (22, 23), and wherein saidcarrier shaft (24) is fixedly connected to the gear housing (2).
 17. Acompact gear system according to claim 14, wherein either the outputshaft (1b) or the gear housing (2b) is connected to a robot frame (32)and the other to a robot arm (31).
 18. A compact gear system accordingto claim 14, wherein either the output shaft (1c) or the gear housing(2c) is connected to a first robot arm (33) and the other to a lever(35) which is rotatably mounted on a second robot arm (31) and providedwith a maneuvering strut (36).
 19. A compact gear system according toclaim 1 in which the first (25) and the second (26) toothed rings havemutually different tooth numbers, wherein the cylindrical toothed rings(19, 20) of respective first (11) and second (12) crown wheels havemutually the same tooth number, and wherein the first toothed rings (17,18) of respective first (11) and second (12) crown wheels have mutuallythe same tooth number.
 20. A compact gear system according to claim 19,wherein the offset shaft (21) is hollow and is rotatably journalled on acarrier shaft (24) by internal bearings (22, 23), and wherein saidcarrier shaft (24) is fixedly connected to the gear housing (2).
 21. Acompact gear system according to claim 19, wherein either the outputshaft (1b) or the gear housing (2b) is connected to a robot frame (32)and the other to a robot arm (31).
 22. A compact gear system accordingto claim 19, wherein either the output shaft (1c) or the gear housing(2c) is connected to a first robot arm (33) and the other to a lever(35) which is rotatably mounted on a second robot arm (31) and providedwith a maneuvering strut (36).
 23. A compact gear system according toclaim 1, wherein either the output shaft (1b) or the gear housing (2b)is connected to a robot frame (32) and the other to a robot arm (31).24. A compact gear system according to claim 1, wherein either theoutput shaft (1c) or the gear housing (2c) is connected to a first robotarm (33) and the other to a lever (35) which is rotatably mounted on asecond robot arm (31) and provided with a maneuvering strut (36).
 25. Acompact gear system according to claim 1, wherein the offset shaft (21)is hollow and is rotatably journalled on a carrier shaft (24) byinternal bearings (22, 23), and wherein said carrier shaft (24) isfixedly connected to the gear housing (2).
 26. A compact gear systemaccording to claim 25, wherein either the output shaft (1b) or the gearhousing (2b) is connected to a robot frame (32) and the other to a robotarm (31).
 27. A compact gear system according to claim 25, whereineither the output shaft (1c) or the gear housing (2c) is connected to afirst robot arm (33) and the other to a lever (35) which is rotatablymounted on a second robot arm (31) and provided with a maneuvering strut(36).